XDSL is a technology that overcomes some of the limitations of plain old telephone service (“POTS”). The acronym XDSL generally identifies the broad class of “digital subscriber line” services. The letter “X” in the general acronym can be replaced with another letter to identify a specific type of digital subscriber line service, such as asymmetric digital subscriber line (“ADSL”) service or high-bit-rate digital subscriber line (“HDSL”). Using XDSL, the bandwidth of a carrier loop can be extended to support higher speed data communications than provided by POTS, and the carrier loop may support simultaneous voice and data communications.
The carrier loop can be formed by connecting a line card, generally located at a central office, with a far end unit. The far end unit may be, for example, an XDSL device. The central office can generally house one or more network interfaces. A network interface may include multiple line cards, each of which may connect to a different far end unit.
Various problems, such as faults, may occur within an XDSL transmission system. For example, physical problems may develop on the connections between the far end units and the line cards. In another example, one or more components in the XDSL system may physically fail. Other problems can also develop.
In order to prevent a problem on one carrier loop from affecting the other carrier loops, the network interface typically provides a complete set of circuitry for each line card. For example, the network interface may have separate control circuits and power supplies for each line card. That way, if a control circuit or power supply fails, it will not affect the operation of the other line cards. Also, a failure in a carrier loop will not be reflected back through the common circuitry to the other carrier loops. While this provides some degree of protection against transmission problems, the use of separate circuitry for each line card increases the total number of components in the network interface. This can increase both the cost and complexity of the network interface.
In another method of providing fault tolerance in an XDSL transmission system, the network interface may be matched with a corresponding standby interface. In the event of a problem in one or more of the carrier loops, the XDSL system can switch from using the network interface to using the standby interface. This implementation, however, has disadvantages. In addition to the carrier loop having the problems, other non-affected carrier loops are also switched from using the network interface to using the standby interface. In order to continue communication with the far end units, the line cards in the standby network interface should be initialized with the far end units. For example, according to some current XDSL standards, the initialization process can be on the order of thirty seconds. Therefore, the carrier loops that are unaffected by the problem in the XDSL system may still experience an outage of service greater than thirty seconds while the far end units initialize with the new line cards in the standby interface.
Therefore, there exists a need to provide an improved method for providing fault tolerance in an XDSL system.